This invention relates to nuclear reactors and, more particularly, to a method for repairing bent mixing vanes found in the fuel assemblies of nuclear, pressurized-water reactors.
As described in co-assigned U.S. Pat. No. 3,719,560 issued to MAYERS et al., located within a conventional nuclear, pressurized-water reactor is a plurality of parallel fuel rod supporting grids. These grids are made up of a plurality of thin bands or straps arranged in a lattice configuration to form individual fuel cells. Each fuel rod containing nuclear fuel pellets is inserted through a fuel cell within a grid. Each fuel rod is held in a fixed relationship within the fuel cell by a plurality of springs (see, e.g., co-assigned U.S. Pat. No. 3,379,618 issued to FRISCH), and punched, metal dimples arranged along the inner faces of each fuel cell. Each fuel rod is supported by being lightly pushed against the dimples via one or more of the springs.
The grids also include metal fins or "mixing vanes" at the corners thereof, perpendicular to the grid straps, which may lightly abut, but do not mechanically support, the fuel rod. Mixing vanes are intended to disturb fluid flow, i.e., cause a swirling action to improve heat transfer and reduce the potential for hot spot temperatures at the fuel rods.
During individual fuel rod removal and reassembly, e.g., at scheduled maintenance, the mixing vanes can be damaged, i.e., bent. A bent mixing vane interferes with normal fuel rod reinsertion, i.e., exerts bending moment on the fuel rod, thus causing the fuel rod to deflect and bow out of its intended path. Deflection causes the fuel rod to "hang-up" on the first grid below the grid with the bent mixing vane.
If a fuel rod is hung up, it has been usual practice to remove the fuel rod and not to replace it. As a result, a void is left where fluid can leak through. The performance of the fuel assembly is degraded as a result of deleting one or a plurality of fuel rods which cannot be reinserted.
In light of the above, a method for straightening bent mixing vanes is desired so that fuel rods can be reliably reinserted.